The present invention generally relates to a semiconductor device, and more particularly relates to a high electron mobility transistor (HEMT) used in a millimeter wave band.
In recent years, multimedia applications have rapidly and tremendously broadened in order to catch up with the up-to-date trend of the present information-oriented society. As a result, in the field of mobile communication, large-capacity high-speed communication is now in very high demand. This is because the times now require the transmission of a large quantity of data, like moving pictures, at a much higher speed, to say nothing of still pictures or quasi-moving pictures. In the bands of 0.3 GHz to 3 GHz, currently used by cellular phones and personal handy phone systems (PHS), there are no longer sufficient frequency resources accommodated to meeting such a demand, that is, large-capacity high-speed communication. Thus, the use of a millimeter wave band, having a very wide frequency range of 30 to 300 GHz, is now under research and development. In order to utilize a millimeter wave band, an active device, attaining a sufficiently high gain and exhibiting excellent low-noise characteristics in the frequency range in question, is required. And a semiconductor layer, where electrons reach a higher saturation velocity, should be used as the electron transportation layer (i.e., a channel layer). Accordingly, it was reported that if an InAlAs/InGaAs HEMT, having a channel layer made of InGaAs with a high saturation velocity, is used, then a maximum oscillation frequency fmax of more than 600 GHz, which belongs to the highest frequency band for active devices, is attained.
However, this InAlAs/InGaAs HEMT also has a problem that the contact resistance of an ohmic contact layer deteriorates with the passage of time (or with the application of heat).
In accordance with one suggested prior art technique, a refractory metal such as tungsten (see, e.g., Jpn. J. Appl. Phys. Vol. 33 (1994), pp. 3373-3376) or molybdenum (see, e.g., Technical Report of Institute of Electronics, Information and Communication Engineers in Japan, ED93-133, CPM93-104 (1993-11), pp. 77-82) is used as a material for an ohmic electrode to prevent this deterioration with time. In general, refractory metals are thermally stable and do not cause counter diffusion with indium (In). However, these refractory metals are ordinarily provided by sputtering during a fabrication process. Accordingly, damage is more likely to be induced in semiconductor crystals and it is not easy to form an ohmic electrode with small contact resistance. Also, molybdenum (Mo) is oxidized very easily, though Mo is formed by an evaporation technique not inducing so much damage. Thus, the metal should be treated very carefully during a fabrication process. For example, a wet treatment should be avoided after the electrode has been made of Mo.
According to another suggested prior art technique, a GaAs layer is formed between a cap layer containing In and an ohmic electrode to protect the cap layer (see, e.g., Japanese Laid-Open Publication No. 6-84958). In accordance with this method, a typical three-layer stack of Ti/Pt/Au may be used as a material for the electrode. Accordingly, this method easily fit in with conventional fabrication processes for GaAs MESFETs and pseudomorphic HEMTs.
In a HEMT formed by the latter prior art technique, a GaAs protective layer is formed between a cap layer and an ohmic electrode. Accordingly, although the contact resistance of the electrode is low immediately after the fabrication thereof, the resistance possibly increases with time depending on the thickness of the protective layer.
In view of these conventional problems, the objects of the present invention are reducing the contact resistance of an ohmic electrode and preventing the increase of the resistance with the passage of time.
In order to accomplish these objects, the thickness of a protective layer made of GaAs and provided between a cap layer made of a semiconductor containing In and an ohmic electrode is optimized according to the present invention.
The semiconductor device of the present invention includes: a cap layer formed on a substrate and made of a semiconductor including In; an ohmic electrode formed over the cap layer; and a protective layer formed between the cap layer and the ohmic electrode and made of n-type GaAs. The protective layer prevents In atoms, contained in the cap layer, from diffusing toward the ohmic electrode and also prevents metal atoms, contained in the ohmic electrode, from diffusing toward the cap layer. The thickness of the protective layer is larger than 5 nm and smaller than 15 nm.
In the semiconductor device of the present invention, the thickness of the n-type GaAs protective layer, formed between the cap layer made of a semiconductor including In and the ohmic electrode, is larger than 5 nm and smaller than 15 nm. Accordingly, neither defects nor dislocations, which are ordinarily found in a protective layer in remarkable numbers, are caused in this protective layer. Thus, the contact resistance can be kept sufficiently low between the cap layer and the ohmic electrode and does not increase with the passage of time. As a result, a semiconductor device, exhibiting stabilized electrical characteristics for a longer period of time, can be obtained.
In one embodiment of the present invention, the protective layer preferably includes an n-type delta doped layer.
In such an embodiment, a well is formed in the conduction band of the n-type delta doped layer. Accordingly, the effective thickness of the energy barrier layer decreases, and therefore tunneling current is even more likely to flow than the case of providing the n-type GaAs protective layer alone. As a result, the contact resistance between the cap layer and the ohmic electrode can be further reduced.
In another embodiment of the present invention, the substrate is preferably a semi-insulating substrate made of InP.
In such an embodiment, a HEMT containing In can be formed with more certainty.
In still another embodiment of the present invention, the substrate is preferably a semi-insulating substrate made of GaAs.
In such an embodiment, a HEMT can be formed at a lower cost.